Optical defects such as myopia, hyperopia, astigmatism and aphakia represent the most common abnormalities of the human eye. The cryorefractive surgical procedures of keratomileusis, keratophakia and epikeratophakia involve the lathing of a patient's cornea, or a donor cornea, to alter the shape and refractive power of the anterior surface of the cornea in order to correct these optical defects. Based on the cornea's preoperative refractive error, corneal tissue is placed on a lathe and frozen, and using computer-directed lathe settings the tissue is resected in a controlled fashion so as to create a lenticule that can be sewn onto or into the patient's cornea to alter the focusing system of the eye.
The cornea consists of several layers: the epithelium (five or six layers of cells which comprise the outer surface of the cornea); a basement membrane from 10 to 30 microns thick directly beneath the epithelial cells; Bowman's membrane (a sheet of acellular transparent tissue below the basement membrane, about 12 microns thick and made up of uniform fibrils of collagenous material; the stroma, further described herein, which is found beyond the Bowman's membrane; Descemet's membrane, a structureless membrane about 10 microns thick which bounds the inner surface of the stroma; and the endothelium, a single layer of cells lining Descemet's membrane which forms the inner surface of the cornea next to the aqueous humor.
The corneal stroma forms the bulk of the cornea. It is a differentiated connective tissue containing 75 to 80% water, and otherwise composed essentially of collagen (a glycoprotein) and glycosaminoglycans (proteoglycans, which are also referred to as mucopolysaccharides). Collagen is a protein having a basic unit molecular weight of about 60 kilodaltons and forms corneal fibrils which constitute the framework of the corneal stroma. Corneal collagen has a high glycine, proline and hydroxyproline content like collagen from other sources.
The glycosaminoglycans are localized in the interfibrillar space and are thought to be charged proteins which, together with the interfibrillar collagen of opposite charge, maintain the proper matrix of fibrils in the cornea. For example, in certain systemic diseases of glycosamino metabolism there is clouding of the cornea, and these glycoproteins are thus thought to be involved in the maintenance of the level of hydration and transparency of the cornea.
The stroma also contains cells (keratocytes) which are responsible for the synthesis of corneal collagen and glycosaminoglycans and thus, in turn, for the maintenance of the clarity of the cornea.
Corneal transparency is explained by the lattice structure formed by the corneal collagen fibrils, which are arranged so that scattering of light is eliminated by mutual interference. As long as the fibrils are regularly arranged in a lattice and separated by less than a wavelength of light, the cornea is transparent. If the arrangement of the fibers is distorted, destructive interference no longer occurs and the cornea becomes hazy.
Although the corneal cryo-surgical techniques have been of substantial benefit in the treatment of myopia, hyperopia, astigmatism and other corneal dysfunction, in all forms of refractive cryolathe surgery there is a substantial delay in visual recovery. Specifically, it is not uncommon following these procedures for a patient to be unable to achieve optimal corrected visual acuity of better than 20/30 until approximately one year following the surgery.
The standard protocol for cryorefractive surgery employs a Barraquer cryolathe, manufactured by Steinway Instrument Co., Inc. of San Diego, Calif. The Barraquer cryolathe freezes tissue by the expansion of carbon dioxide gas, and requires a carbon dioxide pressure of 800 p.s.i., thus producing a freezing rate of about thirty-three degrees centigrade per minute. Prior to this invention, maximum freezing rate was thought necessary for cryorefractive lathing so that the excision, lathing and reimplantation could be accomplished as soon as possible.
According to the present invention, a method for the controlled rate freezing in cryorefractive surgery is provided whereby corneal sections are frozen at a rate of less than about seven degrees per minute to produce substantial improvements in the corneal clarity during post-operative recovery. A freezing rate of less than five degrees/minute is preferred.
All temperatures herein are expressed in degrees centigrade.